TNF alpha in cerebrospinal fluid of meningitis patients reduces astrocytes membrane potential

Cytokines and Virus Proteins: Modulators of Glial Electrophysiological Properties

Cytokines are released during acute and chronic inflammatory diseases of the CNS and activate receptors on glial cells, thereby inducing various effects such as proliferation, expression of major histocompatibility complex genes or secretion of growth factors. Here, we summarize current evidence indicating that K+ currents, Ca 2+ currents, and the activity of ion transporters on astrocytes, microglial cells, and oligodendrocytes are also affected by cytokines. In disease states with associated elevated cytokine titers, such alterations in electrophysiological properties of glial cells might contribute to the patho genesis of neurological symptoms. NEUROSCIENTIST 5:142-146, 1999

Dexamethasone prevents LPS-induced microglial activation and astroglial impairment in an experimental bacterial meningitis co-culture model

We analyzed the effect of dexamethasone on gram-negative bacteria derived lipopolysaccharide (LPS) induced inflammation in astroglial/microglial co-cultures. At the cellular level the microglial phenotype converted to an activated type after LPS incubation. Furthermore, LPS compromised functional astroglial properties like membrane resting potential, intracellular coupling and connexin 43 (Cx43) expression. This change in Cx43 expression was not due to a downregulation of Cx43 mRNA expression. Morphological and functional changes were accompanied by a time-dependent release of inflammation related cytokines. Co-incubation of dexamethasone with LPS prevented these LPS-induced changes within our glial co-culture model. The ability of dexamethasone to reconstitute astrocytic properties and to decrease microglial activation in vitro could be one possible explanation for the beneficial effects of dexamethasone in the treatment of acute bacterial meningitis in vivo.

Divergent roles for tumor necrosis factor-alpha in the brain

Proinflammatory cytokines and chemokines have been implicated in the pathogenesis of several neurological and neurodegenerative disorders. Prominent among such factors is the pleiotropic cytokine, tumor necrosis factor (TNF)-alpha. Under normal physiological conditions, TNF-alpha orchestrates a diverse array of functions involved in immune surveillance and defense, cellular homeostasis, and protection against certain neurological insults. However, paradoxical effects of this cytokine have been observed. TNF-alpha is elicited in the brain following injury (ischemia, trauma), infection (HIV, meningitis), neurodegeneration (Alzheimer's, Parkinson's), and chemically induced neurotoxicity. The multifarious identity for this cytokine appears to be influenced by several mechanisms. Among the most prominent are the regulation of TNFalpha-induced NF-kappaB activation by adapter proteins such as TRADD and TRAF, and second, the heterogeneity of microglia and their distribution pattern across brain regions. Here, we review the differential role of TNF-alpha in response to brain injury, with emphasis on neurodegeneration, and discuss the possible mechanisms for such diverse and region-specific effects.

TNFalpha reduces glutamate induced intracellular Ca(2+) increase in cultured cortical astrocytes

The proinflammatory cytokine TNFalpha is locally released during various inflammatory CNS diseases and high cerebrospinal fluid (CSF) titers of TNFalpha were found in meningitis patients. We know from previous studies that TNFalpha also depolarizes astrocytes by reducing their inwardly rectifying K+ currents. We have now investigated the effect of TNFalpha on the glutamate induced intracellular Ca2+ increase in astrocytes, a process which seems to be involved in glial mediated modulation of neuronal synaptic transmisssion. Incubation with TNFalpha (50-1000 U/ml for 60 min) reduces the glutamate induced intracellular Ca2+ increase in astrocytes but not in neurons and this seems to be a phenomenon secondary to the TNFalpha induced depolarization. While other proinflammatory cytokines (interleukin 1beta, IL-2, IL-6) did not interfere with the astrocytic glutamate response, incubation in CSF from septic meningitis patients (CSF-SM) also reduced the glutamate induced intracellular Ca2+ increase. The application of a neutralizing anti-TNFalpha antibody to the CSF-SM prior to cell incubation partially restored the glutamate response. Our data suggest that inflammatory molecules such as TNFalpha impair astrocytes' response to glutamate and this may indirectly affect neuronal synaptic transmission.

Effects of tumor necrosis factor alpha on taurine uptake in cultured rat astrocytes

Taurine is known to play a major role in volume regulation in astrocytic swelling associated with stroke and brain trauma. Apart from brain edema, the severity of brain injury is related to the levels of inflammatory cytokines such as tumor necrosis factor alpha (TNFalpha). TNFalpha had been shown to be closely associated with brain edema formation since the neutralization of TNFalpha reduced brain edema. Considering taurine has osmoregulatory functions in astrocytes, experiments were performed to study the effects of TNFalpha on taurine uptake in cultured astrocytes. Astrocytes exposed to 20 ng/ml of TNFalpha for 48 h showed a 91% increase in taurine uptake and significant increase was observed after 24 h exposure. This cytokine caused neither significant changes in cell volume nor taurine release. The increased in taurine uptake induced by TNFalpha was unlikely resulted from the modification of Na(+) movement because TNFalpha decreased tyrosine uptake, Na(+)-dependent transport system. In contrast to TNFalpha, interferon-gamma (IFNgamma) did not significantly affect taurine uptake. Taken together, our results did not support a suggestion that TNFalpha affects cell volume regulation via modulating taurine uptake in astrocytes. Increasing lines of evidence have demonstrated that taurine has anti-inflammatory and anti-oxidative effects, these findings therefore suggested that the increase in taurine uptake might be an adaptive response or a tool for astrocytes against oxidative stress.

TNF alpha induces a protein kinase C-dependent reduction in astroglial K+ conductance

Incubation of cultured cortical astrocytes with tumor necrosis factor alpha (TNF alpha) led to a marked reduction of membrane potential. Here we report that this depolarization depends on activation of protein kinase C (PKC), since it could be blocked by the PKC antagonists staurosporine and H7 and it could be mimicked by direct activation of PKC using the phorbol ester phorbol 12-myristate 13 acetate (PMA). Analyses of whole cell currents revealed a reduction of inwardly rectifying K+ currents whereas K+ outward currents were not affected. We conclude that TNF alpha induces changes of basic electrophysiological properties of astrocytes which are similar to those induced by proliferation or an in vitro model of traumatic injury.

Immunologically induced electrophysiological dysfunction: implications for inflammatory diseases of the CNS and PNS

During inflammation of the central or peripheral nervous system, a high number of immunologically active molecules, including bacterial or viral products as well as host-derived cytokines, are released. Patients suffering from inflammatory CNS or PNS diseases often develop transient symptoms with a rapid recovery, which obviously cannot be accounted for by immunologically induced tissue damage. These observations led to the hypothesis that immunologically active molecules can affect directly the electrophysiological functions of neurons and glial cells. Evidence for this hypothesis came from in vitro studies showing that cytokines, such as interleukins or tumor necrosis factors, arachidonic acid and its metabolites, interfere with electrophysiological properties of neurons or glial cells. These molecules affect ion currents, intracellular Ca2+ homeostasis, membrane potentials, and suppress or enhance the induction and maintenance of long-term potentiation. Similarly, virus proteins from human immunodeficiency virus type I were found to alter intracellular Ca2+ concentrations of neurons and astrocytes by modulating either transmitter receptors and channels or membrane transporters. Cerebrospinal fluid from MS patients contains factors which increase Na+ current inactivation and thereby reduce neuronal excitability. Immunoglobulins in sera of patients suffering from multifocal motor neuropathy and from acquired neuromyotonia interfere with nerve fibers, inducing alterations of conduction. Increased knowledge of these mechanisms will help to explain the pathogenesis of neurological symptoms and may provide a rationale for new therapeutic strategies.